Ink compositions, especially those for use with ink jet printers, must satisfy a number of conditions in order to achieve acceptable printing.
The ink composition should possess properties which permit it to be jetted appropriately from an ink jet printer. In ink jet printing, droplets of ink are expelled from nozzles onto a recording medium. The ink should form a meniscus at each nozzle prior to being expelled in the form of a droplet. After a droplet is expelled, additional ink surges to the nozzle to reform the meniscus. Important properties of the ink in this context include viscosity and surface tension.
To achieve acceptable jetting properties, most inks are water based. Generally, highly water soluble dyes are used to avoid problems such as kogation. Kogation refers to the formation of a solid deposit on the surface of the thermal ink printhead heater surface. As a result, the inks tend to suffer from a number of other problems. In particular, poor water fastness results from utilizing highly water soluble dyes. Upon wetting, the ink can easily be redissolved from the paper after printing.
Attempts have been made to improve water fastness by the addition of various additives and surfactants. However, these materials can adversely affect print quality and/or jetting characteristics.
Shibata et al U.S. Pat. No. 4,724,002 discloses heat-sensitive transfer media containing an ink composition comprising a binder and a pigment. The binder may be a wax and the pigment may be an oil-based pigment or dry color which is suspended within the binder to impart a color to the ink. A process for preparing a solid heat-sensitive transfer medium is disclosed as including (1) providing a mixture of pigment and water; (2) adding the mixture of pigment and water to a molten solid wax binder; (3) mixing the molten wax binder and the mixture of pigment and water, in order to remove the water from the pigment and disperse the pigment as particulates into the molten wax binder; (4) separating the water from the molten wax binder with the pigment particulates; (5) blending additives with the molten wax binder with the pigment particulates to form a molten heat-sensitive transfer medium; and (6) cooling the molten heat-sensitive transfer medium.
Terry et al U.S. Pat. No. 3,406,137 discloses a pressure transfer ink formulation comprising a pigment dispersed in a binder. The binder is comprised of waxes, wax soluble dyes, an adhesive resin and polysiloxane.
Duff et al U.S. Pat. No. 4,762,734 discloses an ink donor film comprising in a hydrocarbon solution of wax components and/or polymer components, or a mixture of wax and polymer components, dispersed in a polar phase containing dispersed pigment or dissolved dye. The coloring agent may include a dye and/or pigment wherein the dye is an alcohol or hydrocarbon-soluble dye. An emulsion process for preparing thermal ink donor films is comprised of: (1) adding wax and/or polymer components to a hydrocarbon solution; (2) adding additives which are soluble in the solution; (3) forming an emulsion by adding to the solution a polar liquid containing a pigment and optional additives; (4) grinding the resulting emulsion; (5) coating the emulsion; and (6) heating the resulting emulsion which has been coated.
Merritt et al U.S. Pat. Nos. 4,390,369 and 4,484,948 disclose a natural-wax ink jet ink containing at least one natural wax. The wax may be used as a basic fluid vehicle or as an additive to other fluid vehicles. A coloring agent or dye such as an oil or solvent dye may be added to the composition.
Hayashi et al U.S. Pat. No. 4,636,258 discloses an ink composition comprising a copolymer, a colorant dispersed therein, and wax or resin. The wax may be added to the colorant when dispersed in the copolymer.
Tabayashi et al U.S. Pat. No. 4,878,946 discloses a hot melt ink for thermal ink-jet printers which comprises an oil-soluble dye and additives dissolved in at least one compound that is solid at ambient temperature.
Trimble et al U.S. Pat. No. 3,353,974 discloses a homogeneous composition for printing inks and various coatings comprising an oil, wax and/or resinous base, and a high concentration of dispersed pigments.
There is a need for ink compositions for ink jet printers which not only provide acceptable jetting properties, but which provide good print quality and water fastness when printed.